1. Field of the Invention
The invention relates to cermets and a ceramic discharge lamp in which the cermet is used for the hermetically sealing components.
2. Description of the Related Art
FIG. 1 is a schematic cross section of one example of a conventional ceramic discharge lamp which has a discharge vessel 3 of translucent ceramic with an arc tube part 1 and hermetically sealed tube parts 2 which are joined to the arc tube part 1. In the arc tube part 1, there are a pair of discharge electrodes 4 opposite one another. The discharge electrodes 4 are located in the tip areas of the upholding parts of the electrodes 5. The base parts of the upholding parts of the electrodes 5 are inserted into hermetically sealing components 6. A hermetically sealed arrangement is obtained by fritting-welding of these components 6 in the tube part 2. In FIG. 1, an outer lead 7 is shown inserted into each of the components 6. The ceramic discharge lamp with this arrangement is described, for example, in Japanese patent disclosure document SHO 61-220265.
The component 6 in this discharge lamp is made of a conductive cermet, which is obtained by sintering of ceramic powder and metal powder, and is hermetically welded by a glass frit (not shown in the drawing) in the tube part 2.
The ceramic powder for obtaining this cermet is the same material as the translucent ceramic comprising the discharge vessel 3, for example, polycrystalline aluminum oxide powder.
Molybdenum powder or tungsten powder is used as the metal powder. To ensure the conductivity necessary for power supply, the metal component for the entire cermet is contained in a percentage by volume of 30 to 60%.
Metals such as molybdenum and tungsten which are contained in the cermet have a smaller coefficient of linear expansion than a ceramic like aluminum oxide. The coefficient of linear expansion of the cermet which comprises the component 6 is therefore less than the coefficient of linear expansion of the ceramic by itself which comprises the cermet. This means that when using the same material as the translucent ceramic comprising the discharge vessel 3 for the ceramic comprising the cermet, the coefficient of linear expansion of the cermet to be obtained is less than the coefficient of linear expansion of the translucent ceramic comprising the discharge vessel 3.
As a result, the following disadvantage arises when the hermetically sealing components are formed, for example, from a conductive cermet based on aluminum oxide-molybdenum, fritting-welding of these hermetically sealing components on the discharge vessel of translucent aluminum oxide ceramic is performed, and thus a discharge lamp is produced. In particular, either in the cooling process in fritting-welding or soon after use of the lamp commences, i.e., within a few hundred hours after starting of discharge lamp operation, cracks form at the welded sites due to the different coefficients of thermal expansion of the material components of the hermetically sealing components and the discharge vessel.
The invention was devised to eliminate the above described defect in the prior art. Therefore, a first object of the invention is to devise a cermet which has a suitable coefficient of linear expansion for a hermetically sealing component of a discharge lamp of ceramic which can be easily produced and which inherently has a high hermetically sealing property.
A second object of the invention is to devise a discharge lamp of ceramic in which a sufficiently hermetically sealed arrangement, and thus a long service life, are obtained by hermetically sealing components of the above described cermet.
In these application documents, the expression xe2x80x9caverage coefficient of linear expansionxe2x80x9d means the average value of the coefficient of linear expansion at 25 to 350xc2x0 C.. This coefficient of linear expansion was determined according to Fis R 3102-1978.
The above objects are achieved in accordance with the invention by the cermet having an aluminum oxide component, a silicon dioxide component, a component for modifying the coefficient of linear expansion, which is formed of a metal oxide other than aluminum oxide and silicon dioxide, and a metal component with a smaller coefficient of linear expansion than aluminum oxide.
The objects of the invention are, furthermore, advantageously achieved in that the silicon dioxide component is contained in a percentage by volume of 5 to 30% in the above described cermet.
The objects of the invention are, moreover, advantageously achieved in that the average coefficient of linear expansion of the above described cermet is 5.6xc3x9710xe2x88x926 to 7.6xc3x9710xe2x88x926 (1/K).
In a discharge lamp of translucent ceramic which has a discharge vessel with an arc tube part and hermetically sealed tube parts which are joined to the arc tube part, in which in the arc tube part there are a pair of discharge electrodes opposite one another, and in which a hermetically sealed arrangement is obtained by fritting-welding of hermetically sealing components onto the hermetically sealed tube parts, in the hermetically sealing components the base parts of the upholding parts of the electrodes being inserted, on the tips of which the discharge electrodes are located, the object of the invention is, furthermore, achieved in that the hermetically sealing components are formed of the above described cermet.
In a discharge lamp of translucent ceramic which has a discharge vessel with an arc tube part and hermetically sealed tube parts which are joined to the arc tube part, in which furthermore in the arc tube part there are a pair of discharge electrodes opposite one another, and in which a hermetically sealed arrangement is obtained by fritting-welding of cylindrical or disc-shaped hermetically sealing components onto the outer faces of hermetically sealed tube parts, in the hermetically sealing components the base parts of the upholding parts of the electrodes being inserted, on the tips of which the discharge electrodes are located, the object of the invention is, moreover, achieved in that the hermetically sealing components are formed of the above described cermet.
By composition from an aluminum oxide component, a silicon dioxide component, a component for modifying the coefficient of linear expansion which consists of a metal oxide other than aluminum oxide and silicon dioxide, and a metal component with a smaller coefficient of linear expansion than aluminum oxide, a cermet is obtained with a coefficient of linear expansion which is identical or is similar to that the translucent ceramic which is advantageously used as the material of the discharge vessel. The reason for this is that, by modifying the content of the component contained for modifying the coefficient of linear expansion which consists of a metal oxide other than aluminum oxide and silicon dioxide, the coefficient of linear expansion of the cermet to be obtained can be modified.
In particular, by using silicon dioxide as the essential component, at a low sintering temperature, a cermet can be obtained which inherently has to a sufficient degree a higher hermetically sealing property than in the case in which no silicon dioxide is used.
By using the above described cermet for the hermetically sealing components of a ceramic discharge lamp, it is possible to effectively prevent cracks from forming at the locations where fritting-welding to the hermetically sealed tube parts of the discharge vessel took place. Furthermore, a ceramic discharge lamp with a long service life can be obtained. In this case, it is desirable for the hermetically sealed components to be cylindrical or disk-shaped and to be frit-welded with the outer faces of the hermetically sealed tube parts.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention.